In recent years, dental curable compositions comprising curable resins and inorganic fillers have been widely used. These are generally called dental composite resins and utilized for various uses such as direct restorative materials for tooth defective parts due to dental caries, etc., dental crown prosthesis restorative materials such as inlays, crowns, and bridges, materials for constructing anchor tooth for dental crown defective parts, block materials for dental CAD/CAM, etc. These dental curable compositions are preferable which have a higher mechanical strength, color tone compatibility with natural teeth, and a transparency equivalent to natural teeth, and further durability on aged deterioration of mechanical strength, etc, is preferable under severer oral circumstances.
The various properties which such a dental curable composition needs, are greatly affected by compositions, particle sizes, shape, etc. of the inorganic filler. The technological background of the inorganic filler which has been used in the dental curable compositions is shown below.
The initial inorganic filler used in the dental curable compositions was a macro-filler having a mean particle size of approximately 5-30 μm obtained by pulverizing massive quartz, silicate glass, etc. Although the dental curable composition containing such a macro-filler was excellent in mechanical strength such as bending strength, the composition has had a poor polishing property, and a defect that the inorganic filler is decreased off due to its abrasion. The submicro filler pulverized to a mean particle size of approximately 0.5-3 μm is used by proceed of trituration technology. However, in the dental curable composition using the submicro filler, realization of its enough high mechanical strength has not been accomplished.
On one hand, a micro filler comprising ultrafine particles having a mean particle size of approximately 0.01-0.05 μm as primary particles, a representative of which is colloidal silicon dioxide obtainable by a combustion hydrolysis method of organosilane compounds, is also used. Although a dental curable composition obtainable by combining the micro filler is excellent in the surface glossiness after polishing, the viscosity of the dental curable composition increases when the micro filler is filled up. When the viscosity of the dental curable composition increases, its filler filling ratio has to be decreased and thereby a higher mechanical strength could not be conferred to the dental curable composition. Moreover, combination of the micro filler also has the defect that the dental curable composition becomes pale opaque according to the scattering phenomenon (Rayleigh scattering) of the light originated from ultrafine particles.
In recent years, research and development on inorganic fillers are proceeded and new types of fillers as described below have been developed. In Patent Document 1, an inorganic filler is proposed which forms independent amorphous layers of a silicon dioxide and at least one kind of the other metal oxides, wherein the silicon dioxide is produced by aggregating a silicon dioxide and at least one kind of the other metal oxides and heat-treating them at a temperature of less than crystallization of the metal oxides. The above-mentioned invention provides a dental composite material which is excellent in lubricating property of a polished surface of the cured material, or color tone compatibility with natural teeth, and which has an opacity over X-rays. However, the inorganic filler of the above-mentioned invention has an insufficient adjustment of pore volume of particles or particle strength, and the dental curable composition which uses the inorganic filler did not have a sufficient mechanical strength.
Patent Document 2 proposes an inorganic filler comprising silicon dioxide and an inorganic oxide other than the silicon dioxide, wherein 5 to 70% by weight of the silicon dioxide is originated from an acidic silicic acid solution, and 30 to 95% by weight of the silicon dioxide is originated from a silica sol. Since the inorganic filler of the above-mentioned invention is amorphous and has X-ray contrast property and the refractive index, pore volume, particle strength, etc. of its particles are adjustable, a porous inorganic filler having pores physically fittable to curable resins can be provided. As a result, since mechanical strength of the dental curable composition comprising the above-mentioned porous inorganic filler is enhanced and the refractive index of its particles can be freely controlled, an excellent transparency equivalent to that of natural teeth can be conferred to the dental curable composition without selecting the kind of the curable resins. However, in Patent Document 2, there were problems that a mechanical strength sufficient as that of a dental curable composition has not been still accomplished, or its durability on aged deterioration was low.
The inorganic filler is silanized by using silane coupling materials in order to improve adaptation to a curable resin. The silane coupling materials have a functional group which forms a chemical bonding with a silanol group of the inorganic filler, and a functional group which forms a chemical bonding with the curable resin. The inorganic filler processed by the silane coupling materials has a good adaptation to the curable resin to increase the filling rate of the dental curable composition to contribute to an improvement in its mechanical strength. In the porous inorganic filler of Patent Document 2, although a physical fitting with the curable resin is obtained, the silanol group for forming a chemical bonding with the silane coupling materials is poor in numbers. Therefore, a sufficient chemical bonding was not obtained between the filler and the coupling materials. As a result, a sufficient mechanical strength for the dental curable composition is not revealed, and it was considered that unreacted coupling materials, or the silanol group, etc. deteriorate by water absorption, and that the durability on aged deterioration of the dental curable composition was low.
Patent Document 3 provides a method for conferring an active isolated silanol group to an inorganic filler which mainly comprises an inactive silicon dioxide. There is exhibited that the inorganic filler enhances chemical bonding with silane coupling materials, and confers an excellent mechanical strength or moisture resistance to its cured body. However, the above-mentioned inorganic filler did not comply with properties such as mechanical strength, and a transparency equivalent to that of natural teeth, which are required of the dental curable composition. Therefore, a dental curable composition has been desired which has a transparency equivalent to that of natural teeth which the conventional dental curable composition has, and has a further improvement of mechanical strength and a durability on aged deterioration.